Production of nb3sn diffusion layers



Oct. 11, 1966 J. L. HAM

PRODUCTION OF NB SN DIFFUSION LAYERS Filed Nov. 21, 1962 WElifi/EAWOKPER? g L.+ FURNACE Fig. I

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INVENTOR.

JOHN L. HAM

United States Patent Oflice 3,277,557 Patented Oct. 11, 1966 3,277,557 PRODUCTEON F Nb Sn DIFFUSION LAYERS John L. Ham, Wellesley Hills, Mass., assignor, by mesne assignments, to National Research Corporation, a corporation of Massachusetts Filed Nov. 21, 1962, Ser. No. 239,254 1 Claim. (Cl. 29-155.5)

The present invention relates to the preparation of coatings of hard superconductor binary alloys. Such coating processes comprise the selection of the higher melting constituent as the substrate to be coated and depositing the other constituent as a thin coating. The coated substrate is then heat treated in a furnace to react the substrate and coating to form a resultant diffusion surface layer of the alloy. The process and resultant product are set forth in the copending application of Allen and Stauffer, S.N. 133,653, filed Aug. 24, 1961.

In such processes, it is important to render the substrate surface wettable so that the coating material wets it completely to form an adherent coating which will react with the substrate before it can be vaporized and driven off by heat treatment.

It is therefore the principal object of this invention to provide a process of coating a metallic substrate with another metal to make the coated article suitable for subsequent heat treatment to form a diffusion surface layer of a binary alloy of the two metals.

The method is particularly applicable to niobium substrates to be coated with tin. The binary alloy layer formed by subsequent heat treatment comprises the hard superconductor Nb Sn.

Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention comprises the coating process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claim.

In the following detailed description, reference is made to the drawings wherein:

FIG. 1 is a schematic representation of a process for coating a niobium wire with tin;

FIG. 2 is a schematic representation of a second embodiment of the process; and

FIG. 3 is a schematic representation of a third embodiment of the process.

Referring now to FIG. 1, there is shown a bath of molten flux and a molten tin bat-h 12. Both are maintained in an inert gas atmosphere, such as argon. One example of a suitable flux would be sodium hydroxide. A niobium wire 14 is passed through the flux bath to remove surface oxides and then is passed through the tin bath. The wire is removed from the bath via a die 16. The die may be made of aluminum oxide or if not exposed to atmosphere, of diamond. Suitable heaters (not shown) are provided to maintain the baths in their molten state. Tin melts at 230 C. and most useful fluxes melt in the range 500-800 C.

The die 16 produces a very slight reduction, on the order of 5%, in the diameter of the wire. Yet the die produces a uniform, and adherent, tin coating on the Wire. This Wire may then be placed in an inert gas furnace and heated up to about 1000 C. to produce a surface diffusion layer comprising Nb Sn. The high superconductivity characteristics of such wire have been demonstrated by Allen and Stauffer in the said copending appli cation.

Referring now to FIG. 2, there is shown a second embodiment of the coating process wherein a tank holds tin bath and a flux bath 114 floating on the bath. A die 116 is provided at the tin bath, as in the first embodiment.

The flotation of the flux on the tin insures against any possibility of contaminating the niobium surface prior to dipping in the tin bath. Heaters 118 are provided to heat the flux and tin to their melting points. The tank 100 is provided with an intermediate neck 120 and cooling fins 122 to maintain the temperature gradient. Part of the flux tends to freeze into a solid layer 124. However, the wire 114 is easily pulled through this layer and the separation of liquid flux from liquid tin avoids an undesired reaction.

Referring now to FIG. 3, there is shown a third embodiment of the process wherein a niobium wire 214 is drawn through inert atmosphere chambers 220, 222 and 224. Inlet and outlet valves 226 are provided. The chamber 222 contains a tin bath 210. The Wire 214 is dipped in this bath and removed via a die 216 which may be made of diamond. A broach 22-8 is employed -to strip a surface layer from the Wire thus exposing a cleaner surface of the wire which is immediately dipped and drawn through the die.

While the invention has been discussed principally in its utility for coating wire, it is also applicable to rod and sheet with appropriate changes in die construction or using rollers to simulate the action of a die.

Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

In the art of heating tin coated wires to produce a surface diffusion layer of Nb Sn, the improvement comprising the process of cleaning a niobium wire to remove surface contaminants therefrom by removing a surface layer of the niobium wire in a broaching operation, passing the wire to a tin bath wherein a surface layer of tin is accreted thereon, removing the wire from the bath via a die adapted to reduce the diameter of the wire to obtain a uniform adherent coating of tin and subsequently heating the reduced composite wire to produce a surface diffusion layer of Nb Sn.

References Cited by the Examiner UNITED STATES PATENTS 2,906,018 9/1959 Baker 29-528 2,950,526 8/1960 Buehler et al. 29528 X 3955, 102 9/1962 Shaw et al. 72--275 X 3,157,093 11/1964 Shaw et al. 72--275 X 3,181,936 5/1965 Denny et al.

JOHN F. CAMPBELL, Primary Examiner.

P. W. COHEN, Assistant Examiner. 

